The present invention relates to an image forming apparatus employing a transfer process for transferring an image to an image receiving medium and a fixing process after the transfer, and more particularly, to an image forming apparatus which is capable of preventing an image unevenness from being caused during image transfer, and of being made compact.
There has been known an image forming method, such as an image recording method of an electrophotographic system, in which an image is first formed on an image formation medium or the image is further transferred from the image formation medium onto an intermediate transfer body from the image formation medium, and then the image is transferred from the image formation medium or the intermediate transfer body onto the image receiving medium (i.e., recording sheet) to be fixed onto the image receiving medium by the application of pressure.
In the image forming method in which an image is transferred as described above, load change occurred in the image receiving medium during the transfer of the image leads to an image unevenness (so-called banding), which deteriorates the quality of the image. Factors responsible for the load change that causes the banding include an impact given to the image receiving medium when the image receiving medium enters a downstream process during transfer (i.e., a forward end entry load), or a difference in speeds between downstream conveyance and transfer.
Specially, in recent years, as disclosed in JP 2004-109860 A (hereinafter referred to as Patent Document 1), for example, a print with quality as high as that of a silver halide photographic print even by the image forming method of an electrophotographic system, by adopting such means as giving a gloss to the image receiving medium.
In the print of high quality, the image unevenness becomes a serious problem, and thus it is significantly important to eliminate the occurrence of banding. However, since a thick image receiving medium is preferred for the print with the high image quality, the forward end entry load is large with the result that banding is liable to occur.
In order to suppress the banding, there have been proposed various methods of reducing or eliminating a load change caused while an image is being transferred onto the image receiving medium.
For example, JP 03-253882 A (hereinafter referred to as Patent Document 2) discloses an image forming apparatus for conducting transfer and fixing or the like with the application of pressure in electrophotography, in which a distance between the transfer position and the fixing position is made larger than a maximum length of the image receiving medium applicable, to thereby reduce deterioration of an image quality due to banding or the like.
Also, JP 58-126561 A (hereinafter referred to as Patent Document 3) discloses an image fixing device using a pressure in which an electromagnetic force is used in addition to a spring to make a pressure force variable. With the use of the above device, it may also be possible to reduce the banding by setting a pressure force to be low at the beginning, and by increasing the pressure force after the image receiving medium has entered the fixing device.
However, as disclosed in Patent Document 2, in order to make the distance (i.e., conveying length) between the transfer position and the fixing position larger than the maximum length of the medium, it is necessary to keep the distance large enough, which leads to an increase in size of the apparatus or an increase in cost. In particular, in a case of using plural kinds of image receiving media that are different in length, not only is it necessary to make the distance (i.e., conveying length) long between the transfer position and the fixing position, but it is also necessary to further provide conveying means such as a conveyor belt for conveying the image receiving medium that is smaller than the maximum size between the transfer position and the fixing position. This further causes the size or the cost of the apparatus to increase.
Also, in the fixing of the formed image in the electrophotrographic system, it is necessary to maintain a pressure force at a certain strength. For that reason, when the method of changing the pressure force during fixing disclosed in Patent Document 3 is adopted, the load relationship among the respective processes (i.e., units) varies due to the load change caused at a fixing portion during fixing and conveying, which results in banding.